The introduction of desired agents into specific target cells has been a challenge to scientists for a long time. The challenge of specific targeting of agents is to get an adequate amount of the agent or the correct agent to the target cells of an organism without providing too much exposure of the rest of the organism. A very desired target for delivery of specific agents is the selective control of the immune system. The immune system is a complex response system of the body that involves many different kinds of cells that have differing activities. Activation of one portion of the immune system usually causes many different responses due to unwanted activation of other related portions of the system. Currently, there are no methods or compositions for producing the desired response by targeting the specific components of the immune system.
One method that has been used with limited success is the targeting of cells that bear a specific receptor and providing an antibody to that receptor that acts as a carrier for an agent. The agent could be a pharmaceutical agent that is a cell stimulant or the therapeutic agent could be a radioactive moiety that causes cell death. The problems inherent in this techniques are the isolation of the specific receptor, the production of an antibody having selective activity for that receptor and no cross reactivities with other similar epitopes, and attachment of the agent to the antibody. A problem attendant to such limited delivery is that the agent may never be released internally in the targeted cell, the agent is not releasably bound to the antibody and therefore, may not be fully active or capable of any activity once it is delivered to the site.
The immune system is a complex interactive system of the body that involves a wide variety of components, including cells, cellular factors which interact with stimuli from both inside the body and outside the body. Aside from its direct action, the immune system's response is also influenced by other systems of the body including the nervous, respiratory, circulatory and digestive systems.
One of the better known aspects of the immune system is its ability to respond to foreign antigens presented by invading organisms, cellular changes within the body, or from vaccination. Some of the first kinds of cells that respond to such activation of the immune system are phagocytes and natural killer cells. Phagocytes include among other cells, monocytes, macrophages, and polymorphonuclear neutrophils. These cells generally bind to the foreign antigen, internalize it and may. destroy it. They also produce soluble molecules that mediate other immune responses, such as inflammatory responses. Natural killer cells can recognize and destroy certain virally-infected embryonic and tumor cells. Other factors of the immune response include both complement pathways which are capable of responding independently to foreign antigens or acting in concert with cells or antibodies.
One of the aspects of the immune system that is important for vaccination is the specific response of the immune system to a particular pathogen or foreign antigen. Part of the response includes the establishment of “memory” for that foreign antigen. Upon a secondary exposure, the memory function allows for a quicker and generally greater response to the foreign antigen. Lymphocytes in concert with other cells and factors, play a major role in both the memory function and the response.
Generally, it is thought that the response to antigens involves both humoral responses and cellular responses. Humoral immune responses are mediated by non cellular factors that are released by cells and which may or may not be found free in the plasma or intracellular fluids. A major component of a humoral response of the immune system is mediated by antibodies produced by B lymphocytes. Cell mediated immune responses result from the interactions of cells, including antigen presenting cells and B lymphocytes (B cells) and: T lymphocytes (T cells).
The response is initiated by the recognition of foreign antigens by various kinds of cells, principally macrophages or other antigen presenting cells. This leads to activation of lymphocytes, in particular, the lymphocytes that specifically recognize that particular foreign antigen and results in the development of the immune response, and possibly, elimination of the foreign antigen. Overlaying the immune response directed at elimination of the foreign antigen are complex interactions that lead to helper functions, stimulator functions, suppresser functions and other responses. The power of the immune system's responses must be carefully (ntrolled at multiple sites for stimulation and suppression or the response will either not occur, over respond or not cease after elimination.
The recognition phase of response to foreign antigens consists of the binding of foreign antigens to specific receptors on immune cells. These receptors generally exist prior to antigen exposure. Recognition can also include interaction with the antigen by macrophage-like cells or by recognition by factors within serum or bodily fluids.
In the activation phase, lymphocytes undergo at least two major changes. They proliferate, leading to expansion of the clones of antigen-specific lymphocytes and amplification of the response, and the progeny of antigen-stimulated lymphocytes differentiate either into effector cells or into memory cells that survive, ready to respond to re-exposure to the antigen. There are numerous amplification mechanisms that enhance this response.
In the effector phase, activated lymphocytes perform the functions that may lead to elimination of the antigen or establishment of the vaccine response. Such functions include cellular responses, such as regulatory, helper, stimulator, suppressor or memory functions. Many effector functions require the combined participation of cells and cellular factors. For instance, antibodies bind to foreign antigens and enhance their phagocytosis by blood neutrophils and mononuclear phagocytes. Complement pathways are activated and may participate in the lysis and phagocytosis of microbes in addition to triggering other body responses, such as fever.
In the immune response to antigens, immune cells interact with each other by direct cell to cell contact or indirect cell to cell (factor mediated) communication. For example, interactions between T cells, macrophages, dendritic cells, and B cells are necessary for an effective immune response. B and T cells are activated by signals from dendritic cells or macrophages, which are antigen presenting cells (APC) that present antigens and deliver activation signals to resting cells. Activated T cells help control immune responses and participate in the removal of foreign organisms. Helper T cells cause cells to become better effector cells, such as helping cytotoxic T cell precursors, to develop into killer cells, helping B cells make antibodies, and helping increase functions of other cells like macrophages. Activated B cells divide and produce antigen specific antibodies and memory B cells. The cells involved in the immune response also secrete cellular factors or cytokines, which enhance the functions of phagocytes, stimulate inflammatory responses and effect a variety of cells.
The reactions of these cells also involve feedback loops. Macrophages and other mononuclear phagocytes, or APCs, actively phagocytose antigens for presentation to B and T cells and such activity can be enhanced by lymphocytic cellular factors. Macrophages also produce cytokines that, among other activities, stimulate T cell proliferation and differentiation, and that recruit other inflammatory cells, especially neutrophils, and are responsible for many of the systemic effects of inflammation, such as fever. One such cytokine, called interleukin-12, is especially important for the development of cell-mediated immunity.
Dendritic cells are also APCs which initiate an immune response. There are a number of different types of dendritic cells, including lymphoid dendritic cells and Langerhans cells of the skin. They can be found throughout the body and particularly in the spleen, lymph nodes, tonsils, Peyer's patches, and thymus. They are irregularly shaped cells which continuously extend and contract dendritic (tree-like) processes. One of their roles in the immune system is to regulate and induce B and T cell activation and differentiation. They are potent accessory cells for the development of cytotoxic T cells, antibody formation by B cells, and some polyclonal responses like oxidative mitogenesis. They also stimulate T cells to release the cytokine interleukin-2.
An important arm of vaccination is the response to antigens that is provided by B lymphocytes or B cells. B cells represent about 5 to 15% of the circulating lymphocytes. B cells produce immunoglobulins, IgG, IgM, IgA, IgD, and IgE, which may be released into body fluids, secreted with attached proteins or be inserted into the surface membrane of the B cell. Such immobilized immunoglobulins act as specific antigen receptors. In responding to antigen, these immunoglobulin receptors are crosslinked, known as capping, followed by internalization and degradation of the immunoglobulin. Capping also occurs with glycoproteins located on the surface membrane of the B cells.
The B plasma cells produce and secrete antibody molecules that can bind foreign proteins, polysaccharides, lipids, or other chemicals in extra cellular or cell-associated forms. The antibodies produced by a single plasma cell are specific for one antigen. The secreted antibodies bind the antigen and trigger the mechanisms that facilitate their destruction.
Monoclonal Antibodies
One of the most widely employed aspects of the immune response capabilities is the production of monoclonal antibodies. The advent of monoclonal antibody (Mab) technology in the mid 1970s provided a valuable new therapeutic and diagnostic tool. For the first time, researchers and clinicians had access to unlimited quantities of uniform antibodies capable of binding to a predetermined antigenic site and having various immunological effector functions. Currently, the techniques for production of monoclonal antibodies is well known in the art.
These monoclonal antibodies were thought to hold great promise in medicine and diagnostics. Unfortunately, the development of therapeutic products based on these proteins has been limited because of problems that are inherent in monoclonal antibody therapy. For example, most monoclonal antibodies are mouse derived and, thus, do not fix human complement well. They also lack other important immunoglobulin functional characteristics when used in humans.
The biggest drawback to the use of monoclonal antibodies is the fact that nonhuman monoclonal antibodies are immunogenic when injected into a human patient. After injection of a foreign antibody, the immune response mounted by a patient can be quite strong. The immune response causes the quick elimination of the foreign antibody, essentially eliminating the antibody's therapeutic utility after an initial treatment. Unfortunately, once the immune system is primed to respond to foreign antibodies, later treatments with the same or different nonhuman antibodies can be ineffective or even dangerous because of crossreactivity.
Mice can be readily immunized with foreign antigens to produce a broad spectrum of high affinity antibodies. However, the introduction of murine antibodies into humans results in the production of a human-anti-mouse antibody (HAMA) response due to the presentation of a foreign protein in the body. Use of murine antibodies in a patient is generally limited to a term of days or weeks Longer treatment periods may result in anaphylaxis. Moreover, once HAMA has developed in a patient, it often prevents the future use of murine antibodies for diagnostic or therapeutic purposes.
To overcome the problem of HAMA response, researchers have attempted several approaches to modify nonhuman antibodies, to make them human-like. These approaches include mouse/human chimers, humanization, and primatization. Early work in making more human-like antibodies used combined rabbit and human antibodies. The protein subunits of antibodies, rabbit Fab fragments and human Fc fragments, were joined through protein disulfide bonds to form new, artificial protein molecules or chimeric antibodies.
Recombinant molecular biological techniques have been used to create chimeric antibodies. Recombinant DNA technology was used to construct a gene fusion between DNA sequences encoding mouse antibody variable light and heavy chain domains and human antibody light chain (LC) and heavy chain (HC) constant domains to permit expression of chimeric antibodies. These chimeric antibodies contain a large number of nonhuman amino acid sequences and are immunogenic to humans. Patients exposed to these chimeric antibodies produce human-antichimera antibodies (HACA). HACA is directed against the murine V region and can also be directed against the novel V-region/C-region (constant region) junctions present in recombinant chimeric antibodies.
To overcome some of the limitations presented by the immunogenicity of chimeric antibodies, molecular biology techniques are used to created humanized or reshaped antibodies. The DNA sequences encoding the antigen binding portions or complementarity determining regions (CDRs) of murine monoclonal antibodies are grafted, by molecular means, on the DNA sequences encoding the frameworks of human antibody heavy and light chains. The humanized Mabs contain a larger percentage of human antibody sequences than do chimeric Mabs. The end product, which comprises approximately 90% human antibody and 10% mouse antibody, contains a mouse binding site on an human antibody. It also contains certain amino acid substitutions from the mouse Mab into the framework of the humanized Mab in order to retain the correct shape, and thus, binding affinity for thetarget antigen.
In practice, simply substituting murine CDRs for human CDRs is not sufficient to generate efficacious humanized antibodies retaining the specificity of the original murine antibody. There is an additional requirement for the inclusion of a small number of critical murine antibody residues in the human variable region. The identity of these residues depends upon the structure of both the original murine antibody and the acceptor human antibody. It is the presence of these murine antibody residues that helps create a HACA response in the patient, leading to rapid clearance of the monoclonal antibodies and the fear of anaphylaxis.
Another technique, called resurfacing technology, is used for humanizing mouse antibodies. Resurfacing involves replacing the mouse antibody surface with a human antibody surface in a process that is faster and more efficient than other humanization techniques. This technique provides a method of redesigning murine monoclonal antibodies to resemble human antibodies by humanizing only those amino acids that are accessible at the surface of the V-regions of the recombinant Fv. The resurfacing of murine monoclonal antibodies may maintain the avidity of the original mouse monoclonal antibody in the reshaped version, because the natural framework-CDR interactions are retained. Again, these antibodies suffer from the problem of being antigenic due to their mouse origins.
Other technologies use primate, rather than mouse, sequences to humanize Mabs. The rationale of this approach, called primatization, is that most of the sequences in the primate antibody variable region are indistinguishable from human sequences. Primatized anti-CD4 Mabs for the treatment of rhumatoid arthritis and severe asthma are being developed. However, these Mabs are still foreign proteins to the immune system of the patient and evoke an immune response.
In an effort to avoid the immune response to foreign proteins, a variety of approaches are being developed to make human Mabs that contain only human antibody components. One approach is to isolate a human B cell clone that naturally makes antibody to the desired antigen and grow it in a trioma cell culture system. Because human antibodies are made only against antigens that are foreign to thehost, none of the human B cells will make antibodies against human antigens. Therefore, this approach is not useful to produce Mabs against antigens that are human proteins.
Two other approaches to create human Mabs are phage display and use of transgenic mice. Phage display technique takes advantage of the ability of humans to make antibodies against any possible structure. This technique uses the antibody genes from many individual humans to create a large library of phage antibodies, each displaying a functional antibody variable domain on its surface. From this library, individual variable domains are selected for their ability to bind to the desired antigen. The Mab is created through molecular biology techniques by combining an antibody variable domain having the desired binding characteristics and a constant domain that best meets the potential human therapeutic product. Again, this technique lacks antigen specificity. The phage library cannot contain every binding region for any and all desired antigens. It also may contain binding regions which lack specificity. Thus, this technique may require considerable engineering to increase antibody affinities to useful levels.
Transgenic mice are also being used to create “human” antibodies. The transgenic mice are created by replacing mouse immunoglobulin gene loci with human immunoglobulin loci. This approach may provide advantages over phage display technologies because it takes advantages of mouse in vivo affinity maturation machinery.
All of the current technologies for producing human or human-like Mabs are insufficient to provide a species specific antibody that is antigen specific for a described antigen. Chimeric antibodies have the advantages of retaining the specificity of the murine antibody and stimulating human Fe dependent complement fixation and cell-mediated cytotoxicity. However, the murine variable regions of these chimeric antibodies can still elicit a HAMA response, thereby limiting the value of chimeric antibodies as diagnostic and therapeutic agents.
Efforts to immortalize human B-cells or to generate human hybridomas capable of producing immunoglobulins against a desired antigen have been generally unsuccessful, particularly with human antigens. Additionally, immune tolerance in humans prevents the successful generation of antibodies to self-antigens.
Vaccine Therapy
Vaccines may be directed at any foreign antigen, whether from another organism, a changed cell, or induced foreign attributes in a normal “self” cell. The route of administration of the foreign antigen can help determine the type of immune response generated. For example, delivery of antigens to mucosal surfaces, such as oral inoculation with live polio virus, stimulates the immune system to produce an immune response at the mucosal surface. Injection of antigen into muscle tissue often promotes the production of a long lasting IgG response.
Vaccines may be generally divided into two types, whole and subunit vaccines. Whole vaccines may be produced from viruses or microorganisms which have been inactivated or attenuated or have been killed. Live attenuated vaccines have the advantage of mimicking the natural infection enough to trigger an immune response similar to the response to the wild-type organism. Such vaccines generally provide a high level of protection, especially if administered by a natural route, and some may only require one dose to confer immunity. Another advantage of some attenuated vaccines is that they provide person-to-person passage among members of the population. These advantages, however, are balanced with several disadvantages. Some attenuated vaccines have a limited shelf-life and cannot withstand storage in tropical environments. There is also a possibility that the vaccine will revert to the virulent wild-type of the organism, causing harmful, even life-threatening, illness. The use of attenuated vaccines is contraindicated in immunodeficient states, such as AIDS, and in pregnancy.
Killed vaccines are safer in that they cannot revert to virulence. They are generally more stable during transport and storage and are acceptable for use in immunocompromised patients. However, they are less effective than the live attenuated vaccines, usually requiring more than one dose. Additionally, they do not provide for person-to-person passage among members of the population.
Production of subunit vaccines require knowledge about the epitopes of the microorganism or cells to which the vaccine should be directed. Other considerations in designing subunit vaccines are the size of the subunit and how well the subunit represents all of the strains of the microorganism or cell. The current focus for development of bacterial vaccines has shifted to the generation of subunit vaccines because of the problems encountered in producing whole bacterial vaccines and the side effects associated with their use. Such vaccines include a typhoid vaccine based upon the Vi capsular polysaccharide and the Hib vaccine to Haemophilus influenzae. 
Other vaccines which have been developed include combination vaccines and DNA vaccines. An example of a combination vaccine is the Bordetella pertussis toxin and its surface fimbrial hemaglutinin. In DNA vaccination, the patient is administered nucleic acids encoding a protein antigen that is then transcribed, translated and expressed in some form to produce strong, long-lived humoral and cell-mediated immune responses to the antigen. The nucleic acids may be administered using viral vectors or other vectors, such as liposomes.
The immune response created by vaccines can be non-specifically enhanced by the use of adjuvants. These are a heterogeneous group of compounds or carrier components, such as liposomes, emulsions or microspheres, with several different mechanisms of action.
In addition to the typical use of vaccines for protection against disease, vaccination is being used to fight cancer. The idea of non-specifically stimulating the immune system to reject tumors is nearly a century old. Coley, an early researcher in the field, used bacterial filtrates with considerable success. Attempts to vaccinate against cancer with purified cytokines and immunostimulants have had only limited success and have been effective for only a few types of tumors.
Many diseases, in addition to cancer, are mediated by the immune system. The diseases include allergies, eczema, rhinitis, urticaria, anaphylaxis, transplant rejection, such as kidney, heart, pancreas, lung, bone, and liver transplants; rheumatic diseases, systemic lupus erthematosus, rheumatoid arthritis, seronegative spondylarthritides, sjogren's syndrome, systemic sclerosis, polymyositis, dermatomyositis, type 1 diabetes mellitus, acquired immune deficiency syndrome, Hashimoto's thyroiditis, Graves' disease, . Addison's disease, polyendocrine autoimmune disease, hepatitis, sclerosing cholangitis, primary biliary cirrhosis, pernicious anemia, coeliac disease, antibody-mediated nephritis, glomerulonephritis, Wegener's granulomatosis, microscopic polyarteritis, polyarteritis nodosa, pemphigus, dermatitis herpetiformis, psoriasis, vitiligo, multiple sclerosis; encephalomyelitis, Guillain-Barre syndrome, myasthenia gravis, Lambert-Eaton syndrome, sclera, episclera, uveitis, chronic mucocutaneous candidiasis, Bruton's syndrome, transient hypogammaglobulinemia of infancy, myeloma, X-linked hyper IgM syndrome, Wiskott-Aldrich syndrome, ataxia telangiectasia, autoimmune hemolytic anemia, autoimmune thrombocytopenia, autoimmune neutropenia, Waldenstrom's macroglobulinem.ia, amyloidosis, chronic lymphocytic leukemia, and non-Hodgkin's lymphoma.
Because of the safety concerns associated with the use of attenuated vaccines and the low efficacy of killed vaccines, there is a need in the art for compositions and methods that enhance vaccine efficacy. There is also a need in the art for compositions and methods of enhancing the immune system which stimulate both humoral and cell-mediated responses. There is a further need in the art for the selective adjustment of an immune response and manipulating the various components of the immune system to produce a desired response. Additionally, there is a need for methods and compositions that can accelerate and expand the immune response for a more rapid response in activation. There is an increased need for the ability to vaccinate populations, of both humans and animals, with vaccines that provide protection with just one dose.
What is needed are compositions and methods for target specific delivery of agents to only the target cells. It would be preferable for some administrations and treatments if the agent is internalized by the targeted cells. Once inside the cell, the agent should be sufficiently released from the transport system such that the agent is active. Such compositions and methods should be able to deliver therapeutic agents to the target cells efficiently. What is also needed are compositions and methods that can be used both in in vitro and in vivo systems.
There is also a general need for compositions of antigen specific, species specific antibodies and improved methods for producing them. There is a particular need for methods for producing completely human antibodies having affinity for a predetermined antigen. These human immunoglobulins should be easily and economically produced in a manner suitable for therapeutic and diagnostic formulation.